1. Field of the Invention
This invention relates to an electrode material for a lithium secondary battery (lithium rechargeable battery), which is formed from the powder of particles containing a metal such as silicon or tin that forms an alloy with lithium by electrochemical reaction, to an electrode structure including such an electrode material and also to a secondary battery having such an electrode structure.
2. Related Background Art
The fear for warming the surface of the earth due to the so-called hothouse effect because of the increasing ratio of CO2 gas contained in the atmosphere has been pointed out in recent years. Thermoelectric power plants convert the thermal energy obtained by burning fossil fuel into electric energy but it has been made difficult to build new thermoelectric power plants because they give off CO2 gas by a large quantity. To get rid of this problem, leveling the load of thermoelectric power plants, so-called load leveling, by storing the surplus electric power that is produced during the night in secondary batteries installed in ordinary houses for the daytime when electric power is consumed at an enormous rate has been proposed in order to effectively use the electric power generated by the generators of power plants including thermoelectric power plants.
Besides, development of secondary batteries with a high energy density is expected for electric automobile applications that are characterized by not emitting substances that contaminate the atmosphere such as CO2, NOx and hydrocarbons. Additionally, development of small, lightweight and high performance secondary batteries is an urgent issue for power source applications in the field of portable appliances such as notebook-sized personal computers, video cameras, digital cameras, cellular phones and PDAs (personal digital assistants).
So-called rocking chair type “lithium ion batteries” have been developed and made commercially available for family use as compact, lightweight and high performance secondary batteries. A rocking chair type lithium ion battery is prepared by using a lithium intercalation compound that de-intercalates lithium ions by a reaction that takes place at the time of charging as a positive electrode material and a carbon material, which is typically graphite that can intercalate lithium between two layers of the planes of the carbon hexagons formed by carbon atoms as a negative electrode material.
However, the above “lithium ion battery” cannot realize a secondary battery with a high energy density that is comparable to a lithium primary battery using metal lithium as a negative electrode material because maximally only ⅙ of a lithium atom per carbon atom can be theoretically intercalated into the negative electrode formed from a carbon material. If lithium is tried to be intercalated at a rate higher than the theoretically possible rate into the carbon-made negative electrode of the “lithium ion battery” at the time of charging or if the “lithium ion battery” is tried to be charged with electricity in a high electric current density condition, metal lithium grows on the surface of the carbon-made negative electrode as dendrite (showing a form of ramification) to eventually end up with internal short-circuiting between the negative electrode and the positive electrode as a result of repetition of a charging/discharging cycle. Thus, “lithium ion batteries” having a satisfactory cycle life have not been provided if the graphite electrode is made to show a capacity that exceeds the theoretically possible capacity level.
Meanwhile, high capacity lithium secondary batteries having a negative electrode made of metal lithium have been attracting attention as secondary batteries showing a high energy density but have not achieved any commercial success yet because the cycle life of such batteries is short. The reasons for such a very short cycle life are believed to include that metal lithium reacts with impurities such as moisture in the electrolyte solution and the organic solvent to form an insulating film and that the surface of the metal lithium foil is not plane but has spots where an intense electric field is found to grow dendrite of metal lithium that by turn give rise to internal short-circuiting between the negative electrode and the positive electrode.
There have been proposed techniques to use an alloy of lithium and aluminum for the negative electrode in order to suppress the progress of the reaction between metal lithium and moisture in the electrolyte solution and the organic solvent, which is one of the problems of secondary batteries having a negative electrode made of metal lithium. However, the lithium alloy is very hard and cannot be wound in a spiral form and hence it is not possible to prepare spiral type cylindrical batteries. Additionally, the cycle life is not extended as expected and the energy density comparable to a battery having a negative electrode of metal lithium has not been achieved. Thus, such techniques have not been commercially successful so far for these reasons.
In order to solve the above-described problems, the inventors of the present invention have proposed negative electrodes made of silicon, tin and the like for lithium secondary batteries in U.S. Pat. Nos. 6,051,340, 5,795,679 and 6,432,585 and Japanese Patent Application Laid-Open Publication Nos. H11-283627, 2000-311681 and International Publication No. WO00/17949. More specifically, U.S. Pat. No. 6,051,340 proposes a lithium secondary battery that uses a negative electrode where an electrode layer is formed by using metals of silicon and tin that form an alloy with lithium and metals of nickel and copper that do not form any alloy with lithium on a current collector of a metal material that does not form any alloy with lithium either. U.S. Pat. No. 5,795,679 proposes a lithium secondary battery that uses a negative electrode formed from a powdery alloy of elements such as nickel or copper and elements such as tin. U.S. Pat. No. 6,432,585 proposes a lithium secondary alloy that uses a negative electrode of which the electrode material layer contains particles of silicon and tin having an average particle diameter of 0.5 to 60 μm at an amount of 35 wt % or more to show a void ratio from 0.10 to 0.86 and a density from 1.00 to 6.56 g/cm3. Japanese Patent Application Laid-Open No. H11-283627 proposes a lithium secondary battery that uses a negative electrode containing silicon and tin having an amorphous phase. Japanese Patent Application Laid-Open No. 2000-311681 proposes a lithium secondary battery that uses a negative electrode made of particles of amorphous tin and an alloy of a transitional metal to show a non-stoichiometric composition. International Publication No. WO00/17949 also proposes a lithium secondary battery that uses a negative electrode made of particles of amorphous silicon and an alloy of a transitional metal to show a non-stoichiometric composition.
Japanese Patent Application Laid-Open No. 2000-215887 proposes a lithium secondary battery that has a high capacity and shows a high charging-discharging efficiency achieved by forming a carbon layer on the surfaces of metal or half-metal particles, particularly silicon particles, that can form an alloy with lithium by a chemical deposition process involving thermal decomposition of benzene to raise the thermal conductivity and suppress the expansion of the volume when forming the lithium alloy for the purpose of preventing destruction of the electrode. However, the above cited proposal for a lithium secondary battery using such an alloy type negative electrode is accompanied by drawbacks including that the surfaces of silicon particles cannot be coated uniformly by a chemical deposition process involving thermal decomposition and that the thermal decomposition temperature is high and apt to give rise to oxidation of silicon particles. Therefore, the problem that the internal resistance increases as the charging/discharging cycle is repeated, and consequently the rate of taking out electricity gradually falls is not dissolved sufficiently in comparison with a lithium secondary battery having a graphite electrode.
The negative electrode of a lithium secondary battery that is formed by using powder of a metal selected from silicon, tin and an alloy thereof that can store and discharge lithium by electrochemical reaction and a binder expands as the battery is charged, and the negative electrode contracts as the battery is discharged. Then, as the expansion/contraction cycle is repeated, the contact of the metal particles is decreased to allow metal particles to fall and the current collector to peel off from the electrode layer probably because the reaction of forming an alloy of metal particles and lithium unevenly takes place during charging. Although attempts have been made to improve the uneven reaction and make it more uniform by mixing the material of the negative electrode with carbon particles such as graphite particles, the electrochemical reaction relating to the charging/discharging operations of the lithium secondary battery does not take place uniformly in the electrode layer because of the difference of electric storage capacity and volume expansion between metal particles and carbon particles in the case of increasing the amount of lithium that is stored in the negative electrode during charging (metal particles are units of metal powder and carbon particles are units of carbon powder).
Thus, there is a demand for development of negative electrodes that can provide a long service lifetime in order to dissolve the above-described problems.